I wanted pvm21 to post this here, because it points out something SO important:
When it comes to erections, as in the mechanical part of it, imo nitric oxide is more important. (If you have venous leakage, this is another issue that usually requires testosterone to heal.) My site appears to be focused on testosterone first and nitric oxide second, but in fact I wish it was the other way around. Both are critical - don't get me wrong - but you can survive pretty well in the 300's if your arteries are in decent shape. In fact, you may not even notice anything.
However, again as pvm21's post points out, testosterone strongly affects it in the libido, mood, cognition and other mental aspects. Young guys are really affected by this.
But, again, young guys in particular can often have decent erections (and, as we've seen on other threads, lift incredible poundages) with low testosterone levels. Happens all the time and it's because they have so much nitric oxide that they can overcome the downregulation of NOS that occurs with low testosterone. Us older guys can't get away with that!